There are a lot of reasons to run, but if general fitness is the goal, I've found that sprinting, and in particular uphill sprinting, is the most effective and time-efficient.
If you really want to go for efficiency sprints on an exercise bike are about as time efficient as you can get. 30 seconds flat out, 30 seconds slow x5 and you are 90% of the way there. The stress on your joints is way less than running too.